ON THE CHEMODYNAMICS OF WOLF-RAYET RING NEBULAE - NGC-6888

We have made a chemodynamical study of the W-R ring nebula NGC 6888 around the WN6 star WR 136. The observations combine, for the first time, high spatial and spectral resolution covering most of the optical range in different zones of the nebula. The spectra show an emission system with three different velocity components: (1) V(LSR) = -64 km s-1, (2) V(LSR) = + 78 km s-1, and (3) V(LSR) = + 18 km s-1. Components (1) and (2) correspond to the blue and red-shifted parts of the shell, and (3) is probably associated to the ambient ionized gas. The spectrum of each component has been extracted in order to study its ionization structure and excitation mechanism. Diagnostic diagrams indicate that the observed spectra are typical of purely photoionized nebulae, without any significant contribution of shock excitation. Chemical abundances of O/H, N/H, and He/H have been derived for each velocity component, finding that while the ambient gas has nearly solar abundances, the expanding shell presents a remarkable O/H deficiency, and strong overabundances of N/H and He/H. We conclude that this nebula is composed mostly of stellar ejecta, possibly produced by an event of very high mass loss or shell ejection at, or very near to, the RSG stage of the progenitor of WR 136 with an initial mass in the range 25-40 M ..